A functional digraph is a directed graph in which each vertex has outdegree one. This terminology is synonymous with functional graph in directed graph usage (Harary and Palmer 1973, p. 263).
Functional Digraph
See also
Directed Graph, Functional GraphExplore with Wolfram|Alpha
References
Harary, F. and Palmer, E. M. "A Survey of Graphical Enumeration Problems." In A Survey of Combinatorial Theory (Ed. J. N. Srivastava). Amsterdam, Netherlands: North-Holland, pp. 259-275, 1973.Cite this as:
Weisstein, Eric W. "Functional Digraph." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/FunctionalDigraph.html